Overall web accuracy is determined by examining the web itself. The guide that is being used and the way that the guide is installed. Regardless of how it is installed any web guide has a limit on its ability to current a lethal direction. This limit exists both in time and space. In other words, a web can only literally shift material, finite distance, also any shift takes some amount of time to accrue no matter how small that time is. Your web identify is any literal movement of material that is unwanted. There are several types of error offset errors which are also known as steady-state errors, step-change errors, and transient errors.
An offset or steady-state error means that your material can be shifted in either direction but will tend to take on a fixed amount of shift during your run.
Step-change errors occur when there is an immediate shift in the edge of the material.